1. Field of the Invention
This invention relates to coal liquefaction and is particularly related to staged catalytic coal hydrogenation and liquefaction process for producing high quality, low boiling hydrocarbon liquid products.
2. Description of Prior Art
The two primary approaches for converting coal to liquid fuels are called direct and indirect coal liquefaction. Direct coal liquefaction (“DCL”) reacts coal in a solvent with hydrogen at high temperatures and pressure to produce liquid fuels. DCL was first developed by Dr. Bergius in Germany in 1913 and used commercially in Germany between 1927 and 1945. However, after World War II, crude oil was widely available at reasonable prices and commercial coal liquefaction was therefore not commercially attractive. As a result, very little liquid fuels sold today are produced using a coal liquefaction process.
Indirect coal liquefaction (“ICL”) involves first gasifying coal to produce a synthesis gas which contains principally carbon monoxide and hydrogen and thereafter processing the gas chemically into a variety of fuels.
Where diesel and gasoline type feeds are desired utilizing ICL, the Fischer-Tropsch process is preferably used. The ICL technology was been commercially applied in the 1920-1940's in Germany and since the 1950's in South Africa. While commercially demonstrated, the ICL technologies are very complex, capital intensive, and have low thermal efficiencies.
In the 1970's and 1980's extensive research and development were conducted for direct coal liquefaction in the United States and world-wide, as oil shortages and high oil prices were experienced. The objectives were to produce transportation fuels from coal to reduce oil imports. The US Department of Energy provided financial and technical support to demonstrate two technologies on a large scale (200 ton/day coal feed). The Exxon Donor Solvent (“EDS”) technology liquefies coal with hydrogen and a hydrogen donor solvent at temperatures of 800-840° F. (427-449° C.) and pressures of 2500-3000 psia (172-207 bars). Process derived distillate coal liquids boiling at 400-700° F. (204-371° C.) are hydrotreated at mild conditions over a fixed bed of hydrotreating catalyst (typically nickel-molybdenum on alumina) and recycled as coal slurry oil. From an Illinois No. 6 coal, liquid yields of over 40 w % on dry ash free (“DAF”) coal were obtained during the 2-year demonstration program.
Additionally, the H-Coal Process was invented by Hydrocarbon Research, Inc. and is generally described in U.S. Pat. Nos. 3,519,553 and 3,791,959. The H-Coal Process uses a single ebullated bed reactor with a hydroconversion catalyst to convert coal to liquid fuels. Product oil (400° F.+, i.e. 204° C.+)) was used to slurry the coal for feeding to the reactor. Coal liquefaction took place at temperatures of 800-875° F. (427-468° C.), and hydrogen partial pressures of 1500-2500 psia (103-172 bars). With Illinois No. 6 coal, liquid yields of greater than 50 w % on DAF coal were achieved during the multi-year year demonstration program at the 200 ton per day H-Coal Pilot Plant in Catlettsburg, Ky. The DCL technologies demonstrated commercial readiness, however, no commercial projects proceeded as oil prices fell and oil supplies increased.
In the 1980's and 1990's research continued at a smaller scale to improve the DCL technologies and reduce investments and operating costs. The Catalytic Two-Stage Liquefaction Process (CTSL) was invented by Hydrocarbon Research, Inc., as described in U.S. Pat. Nos. 4,842,719, 4,874,506, and 4,879,021, to substantially increase the yield of distillate liquids from coal. For Illinois No. 6 bituminous coal, liquid yields were increased from 3 barrels per ton of MAF coal for the single stage H-Coal Process to about 5 barrels per ton of MAF coal for the CTSL Process. This was achieved by dissolving the coal feed at mild conditions while simultaneously hydrogenating the coal recycle solvent and coal liquids produced at temperatures from 600-800° F. (316-427° C.), hydrogen partial pressures of 1500-2500 psia (103-172 bars) in the presence of a hydrogenation catalyst.
The coal is then fed to a direct-coupled second stage reactor operating at higher temperatures of approximately 800-850° F. (427-454° C.) and at similar pressures (1500-2500 psia, i.e. 103-172 bars) with a hydroconversion catalyst, to achieve maximum coal conversion and high distillate liquid yields. The hydrogenation catalyst used for the single-stage and two-stage processes deactivates at these reactor conditions due to the deposition of coke and also soluble metals from the coal feed if present. The catalyst is expensive and its replacement in the ebullated-bed reactors therefore greatly contributes to the high cost of coal liquids produced.
Recognizing this problem, U.S. Pat. No. 3,679,573 (Johnson), disclosed a catalytic two-stage coal liquefaction process in which used catalyst is removed from the second-stage reactor and thereafter recycled to the first stage reactor at approximately the same reactor conditions. This so-called catalyst cascading from the second reactor to the first reactor runs counter to the current of the coal feed direction and reduces the quantity of catalyst addition required to achieve a constant liquid product yield and quality.
Unexpectedly and contrary to the prior art, it was learned that the where the first stage reactor was maintained at least 25° F. lower than the second stage reactor in the CTSL process, the coke deposited on the first stage catalyst is substantially lower than on the second stage catalyst. Moreover, the first stage catalyst activity was substantially higher than the second stage catalyst.
U.S. Pat. No. 4,816,141, McLean et al, teaches that the co-current cascading of hydrogenation catalyst from the first stage reactor to the second stage reactor substantially decreases overall catalyst requirements and reduces the cost of producing liquid fuel from coal.
As international fuel quality specifications have become more stringent, there became a growing need for coal liquids having extremely low levels of contaminants (sulfur, nitrogen), low aromatics content, and high cetane indexes. Progress toward this goal has been made by further hydrotreating and hydrocracking the coal liquids in separate downstream processes with new catalysts to meet the desired product quality.
We now have an improved process for integrating the liquid product hydrotreating stage with the catalytic two stage coal liquefaction process, utilizing a single hydroconversion catalyst cascading from the liquid product hydrotreating stage to the first stage low temperature reactor to the higher temperature second stage reactor. Liquid product qualities are greatly improved to meet current or projected industry specifications without any increase in catalyst addition rates on a coal feed basis. Further, the used catalyst from the process can be regenerated by carbon removal and reused in the process in a similar manner as the fresh makeup catalyst described in this invention disclosure.